This invention relates to a method of producing tin-free steel sheets having improved retorting resistance, and more particularly, to a method of producing electrolytic chromate treated steel sheets having improved retorting resistance as the bonded can-forming material.
Electrolytic chromate treated steel sheets also known as tin-free steel (TFS) of chromium type have improved properties as can-forming material and are regarded as a substitute for tin plates, and the demand for them is increasing in recent years.
Since TFS has metallic chromium and hydrated chromium oxide coatings on the surface, it does not possess sufficient weldability. A can is fabricated from a TFS sheet by bonding the mating edges of a blank with a polyamide adhesive to form a can barrel.
Recently, the extent of application of TFS cans has been further spread. That is, TFS cans are not only used for low-temperature packs prepared by packing contents such as carbonated beverage and beer in cans at relatively low temperatures, but also used for the so-called hot packs prepared by packing contents such as fruit juice and coffee in cans at relatively high temperatures for sterilization. TFS is also used in those cans requiring a high temperature retorting treatment for sterilization at the end of packing. In the latter applications, there often occurred accidents of rupture of can barrels.
This can barrel rupture occurs in bonded TFS cans during hot packing and retoring treatment because hot water penetrates through the polyamide resin layer to deteriorate the interfacial adhesion between the paint film and the TFS substrate to eventually separate the paint film from the TFS.
Research has revealed that sulfuric acid, which was conventionally added to chromium plating baths and electrolytic chromic acid baths, was codeposited in the hydrated chromium oxide coating and this sulfuric acid codeposited was dissolved out during the retorting treatment to give rise to the paint film-TFS substrate interfacial separation. Several proposals were made to avoid sulfuric acid codeposition, for example, by using sulfuric acid-free plating bath, or by eliminating sulfuric acid from an acid pickling solution used in a pre-treatment. However, these techniques had a number of industrial problems in that manufacture efficiency is considerably lowered, product quality is less consistent, and yield is low as compared with the traditional techniques.
It is, therefore, an object of the present invention to eliminate the above-mentioned problems of the prior art and to provide an improved method of producing tin-free steel sheets which do not undergo any interfacial separation between TFS and a paint film during a retorting treatment.
The inventors previously proposed in Japanese Patent Application No. 56-62766 a technique capable of overcoming the problem of sulfuric acid codeposition wherein chromium plating is followed by a reverse electrolysis treatment of anodizing the plated steel sheet in the chromium plating bath solution, and then by an electrolytic chromate treatment in an aqueous chromic acid solution.
Continuing further research, the inventors have found that while the previous method of inserting between the chromium plating step and the electrolytic chromate treatment a reverse electrolysis step of anodizing the plated steel sheet is very effective in improving the retorting resistance of the steel sheet, the best quality is obtained only by a proper choice of the extent of the reverse electrolysis treatment, and this proper extent of the reverse electrolysis treatment can be achieved only by a proper choice of conditions such that the quantities of hydrated chromium oxides present on the steel sheet surface before and after the reverse electrolysis may satisfy a certain relationship.